1. Field of the Invention
This invention relates to an optical switch having an input port on which an optical signal is incident, output ports from which an optical signal is outputted, and a plurality of optical waveguides whose output path of an optical signal branch into two, a propagating path of an optical signal incident from the input port is selected to output the optical signal from a desired output port among the output ports. The invention particularly relates to an optical switch of an optical waveguide type, which enables to easily perform a timing-adjustment of a plurality of optical switch sections.
2. Description of the Related Art
Optical waveguide type optical switches are used in optical communication systems and optical exchange systems. Hitherto, there are various kinds of optical-waveguide-type optical switches, for example, the directional coupler type, the Mach-Zehnder interferometer type, and the carrier injection type ones, which use an electro-optical effect. FIG. 6 shows the configuration of a two-input two-output type optical switch that has one of the most fundamental structures.
As shown in FIG. 6, optical signals are incident to an optical switch section SW from input ports Pi(1) and Pi(2), respectively. Then, an optical waveguide, through which an optical signal propagates, is selected in the optical switch section SW. That is, the propagation path of the optical signal is switched to an optical wave guide that propagates the optical signal to a desired one of output ports Po(1) and Po(2). Thus, an optical signal, which is incident from the input port Pi(1) or Pi(2), is outputted from the desired output port Po(1) or Po(2).
It is difficult for the optical waveguide type optical switch shown in FIG. 6 to output all optical power of an incident optical signal only to a desired output port Po(1) or Po(2). For example, even when the switch section SW selects the path to the output port Po(1), a leakage light is generated in the optical switch section SW. Thus, the optical waveguide type optical switch shown in FIG. 6 has a problem that this leakage light is also outputted from the output port Po(2).
The optical communication system and the optical exchange system require that an extinction ratio (that is, a ratio of optical power in a state, in which an optical signal is outputted, to optical power in a state in which no optical signal is outputted) ranges from 20 dB to 30 dB or more. In other words, it is necessary to reduce the crosstalk between the output ports Po(1) and Po(2). However, the extinction ratio in related optical switches is, for instance, about 100 dB.
Thus, an apparatus having multiple stages of optical switches, each of which is shown in FIG. 6 and is connected to optical switch of another stage by an optical fiber, has been proposed. FIG. 7 shows the configuration of a related optical switch apparatus whose extinction ratio is improved (see, for instance, JP-A-6-82847). In FIG. 7, each of optical switches 11 to 13 is the same as the optical switch shown in FIG. 6, and has an optical switch section that selects a desired path by using the electro-optical effect.
For example, an optical switch section of an optical switch 11 selects one of paths, which are respectively connected to optical fibers F1 and F2, for an optical signal that is incident thereon from an input port Pi (3) or Pi (4). The optical signal transmitted by the optical fiber F1 is incident on an optical switch 12. Then, an optical switch section of the optical switch 12 selects one of a path connected to an output port Po(3) and a dummy path. The optical signal transmitted by the optical fiber F2 is incident on an optical switch 13. Then, an optical switch section of the optical switch 13 selects one of a path connected to an output port Po(4) and a dummy path.
For example, in a case where an optical signal being incident on the input port Pi(3) is outputted from the output port Po(3), the optical switch 11 selects the path connected to the optical fiber F1. Subsequently, the optical switch 12 selects the path connected to the output port Po(3). Also, the optical switch 13 selects the dummy path. Therefore, although leakage light generated in the optical switch 11 is transmitted by the optical fiber F2, the leakage light hardly affects the output port Po(4), because the optical switch 13 selects the dummy path. Consequently, the extinction ratio and the crosstalk can be improved.
JP-A-6-82847 is referred to as a related art.
The optical switches 11 to 13 individually produced on different substrates are connected by the optical fibers F1 and F2 in a multistage arrangement. Thus, the extinction ratio and the crosstalk can be improved.
However, because the plural optical switches 11 to 13 are connected by the optical fibers F1 and F2, it is necessary that each of the distance between the optical switches 11 and 12 and the distance between the optical switches 11 and 13 is at least tens cm. Thus, a large delay time (several nsec) is generated between a moment, at which an optical signal is outputted from the optical switch 11, and a moment at which the optical signal reaches each of the optical switches 12 and 13. Therefore, it is necessary that the optical switch section switches the path in consideration of the delay time. Consequently, the related optical switch apparatus has a problem in that it is difficult for the optical switch section to perform timing adjustment.
For instance, in a case where the timing interval between the incidences of optical signals respectively representing data is short, and where an optical signal representing first data is first outputted from the optical switch 11 and another optical signal representing second data is subsequently outputted therefrom, the optical signal representing the second data may be incident on the optical switch 11 before the switching of the path for the optical signal representing the first data is completed in the optical switch 12 or 13. Therefore, it is necessary to individually control the optical switches 11 to 13 according to the incidence timing of each of the optical signals in high-speed optical signal processing. Thus, the related optical switch apparatus has a problem in that it is difficult for the optical switch to perform timing adjustment.
Additionally, it is very difficult to adjust the lengths of the optical fibers F1 and F2, which respectively connect the optical switch 11 to the optical switches 12 and 13, to the same length. This causes a difference between the delay times respectively corresponding to the optical switches 12 and 13. Therefore, the related optical switch apparatus has a problem in that it is more difficult to adjust timing with which the path is switched by the optical switch section.